1. Field of the Invention
The present invention relates to a semiconductor device and its manufacturing method, and more particularly, to a semiconductor device which uses a ferroelectric film and its manufacturing method.
2. Description of the Related Art
In a semiconductor device in which memory cells are 2-dimensionally arranged, specifically in a semiconductor device comprising memory cells with stacked type structure, miniaturization has been accompanied by a narrower space between the cells, and a ratio (aspect ratio) between a height and a width of the stacked memory cell has been increased. In a semiconductor device such as a ferroelectric random access memory (FeRAM) constituted of a ferroelectric capacitor using a ferroelectric film, when hydrogen penetrates into the capacitor in a manufacturing process after the ferroelectric capacitor is formed, characteristics of the capacitor, especially polarization characteristics of the ferroelectric film, are deteriorated. In order to prevent such deterioration, the capacitor is covered with a hydrogen barrier film, e.g., an aluminum oxide film (Al2O3 film), to prevent the penetration of hydrogen. As the Al2O3 film is generally formed by sputtering, at a stepped portion with a higher aspect ratio, coverage of the Al2O3 film becomes poorer thereof, and the capacitor cannot be sufficiently covered with the Al2O3 film.
FIG. 31 shows an example of a ferroelectric memory according to a conventional art. This example shows a source/drain 16 of a metal-oxide semiconductor field effect transistor (MOS transistor) 10 formed on a semiconductor substrate 1, a ferroelectric capacitor 30, and a contact plug 28 formed in a first interlevel insulator 18 for interconnecting these components. The ferroelectric capacitor 30 has a structure in which a ferroelectric film 34 as a capacitive insulator is interposed between a lower electrode 32 and an upper electrode 36. A second interlevel insulator 40 is formed on the ferroelectric capacitor 30, and a hydrogen barrier film 42 made of an Al2O3 film is formed thereon. For example, as a space 38 between ferroelectric capacitors 30a and 30b becomes narrower, an aspect ratio becomes higher, especially an aspect ratio after the formation of the second interlevel insulator 40 becomes higher. When the Al2O3 film 42 is formed on this structure by sputtering, the Al2O3 film 42 becomes thin in a bottom part of the space 38, and no film is formed in an extreme case. Consequently, an effect of preventing the penetration of hydrogen may become insufficient. Thus, it is desired that a surface of the second interlevel insulator 40 between the ferroelectric capacitors 30 is planarised, and the Al2O3 film as the hydrogen barrier film 42 will be formed with an excellent coverage. The planarised surface of the second interlevel insulator 40 can be realized by chemical-mechanical planarization, but it requires an additional process.
FIG. 32 shows an example of switching charge characteristics with respect to the space between the capacitors of the ferroelectric memory according to the conventional art. As apparent from the drawing, switching charge characteristics are deteriorated when the space between the capacitors becomes less than 0.4 μm. This deterioration occurs when the space between the capacitors becomes narrower and coverage of the hydrogen barrier film becomes poorer, and then hydrogen penetrates into the capacitors during the manufacturing process of the semiconductor device after the formation of the ferroelectric capacitors to deteriorate polarization characteristics thereof.
Jpn. Pat. Appln. KOKAI Publication No. 2003-68987 discloses an example of using a hydrogen barrier film to a ferroelectric capacitor. In this example, an upper electrode 10 of the ferroelectric capacitor C has an area smaller than those of a lower electrode 7 or a capacitive insulator 9 (a ferroelectric film). Consequently, a step is formed at an edge of the ferroelectric capacitor C. In order to reduce the step height, an interlayer film 15 is formed to cover the capacitor C, and a hydrogen barrier film 11 is formed thereon. In this example, no consideration is given to a space between capacitors when a plurality of capacitors are formed close to each other, which will cause a problem in a miniaturized semiconductor device.
Additionally, Jpn. Pat. Appln. KOKAI Publication No. 2003-273328 discloses an example of applying an encapsulation barrier film (hydrogen barrier film) 90 to a ferroelectric memory which comprises a plurality of ferroelectric capacitors 60. In this example, a via hole 115 is formed in an interlevel insulator 70 which covers the ferroelectric capacitor 60, and a plate line 120 is formed to interconnect at least two adjacent ferroelectric capacitors 60. During the formation of the via hole 115, the encapsulation barrier film 90 formed between the adjacent ferroelectric capacitors 60 is removed. As a result, the encapsulation barrier film (hydrogen barrier film) 90 does not cover completely all of the plurality of ferroelectric capacitors 60.
Therefore, in the miniaturized ferroelectric capacitor, it is desired to planarise interlevel insulator between ferroelectric capacitors without adding any steps to the process, and to form the hydrogen barrier film thereon with excellent coverage.
The present invention has been made to solve the aforementioned problems, and objects of the invention are to provide a semiconductor storage device wherein a plurality of ferroelectric capacitors are sufficiently covered with a hydrogen barrier film, and a method of manufacturing the same.